| Optical metasurfaces,assembled of artificial subwavelength-scale nanostructure,can realize special electromagnetic responses that do not exist in traditional natural materials,especially optical metasurfaces based on metal surface plasmon resonances(SPRs),which have attracted widespread attention.Traditional optical devices,such as lenses,wave plates,rely on phase shifts accumulated with light propagation to realize optical beam shaping,control and frequency conversion.Due to the huge volume,traditional optical elements cannot be used in modern integrated optics.People hope to realize beam focusing,polarization transformation and frequency conversion at subwavelength scale with linear and nonlinear metasurfaces instead of traditional optical devices,to further promote the implementation of integrated optical components.Therefore,the focus of current research is the design of ultrathin optical elements with traditional optical functions based on metal metasurfaces.In this thesis,we focus on the nonlinear optical effects of metal nanostructures,and we mainly study on the farfield radiation characteristics,wavefront phase control,and polarization control of second harmonic generation(SHG),and enhancement of third harmonic generation(THG).We hope to demonstrate a new mechanism for nonlinear interaction between light and matter based on metal optical metasufaces.Specific contents are arranged as following:1.Local phase control on nonlinear polarization of structural elements in nonlinear metasurfaces.So far,the main research in nonlinear plasmonics has focused on the intrinsic mechanism and the enhancement of nonlinear responses,but few studies on local phase control in nonlinear regime.We theoretically and experimentally investigate the optical second harmonic(SH)diffraction from metasurfaces based on gold complementary split-ring resonators(CSRRs),and reveal that SHG farfield radiation can be tailored by the array composing of CSRRs and reversed CSRRs.By changing the period of the array and wavelength of fundamental wave(FW),we can change the far field radiation angle of SHG,and realize directional radiation and beam splitting of SHG.Through numerical simulations,we have demonstrated that the generated SH currents are concentrated at the surface of the central metal patch and are asymmetric with respect to the base of a CSRR,thus allowing us to impose the phase change of πon the SH radiation by reversing the CSRR’s orientation.Then,we design and fabricate nonlinear metsurface,composed of CSRRs and the reversed CSRRs and realize beam splitting of SH.The ability to control the phase of the local nonlinearity coupled with the high transmittance at both fundamental and SHG wavelengths makes the CSRRs good candidates for the construction of highly efficient three-dimensional nonlinear metamaterials and suitable for applications in nonlinear beam shaping.2.Continuous phase control on SH from nonlinear metal metasurfaces.For SHG,πphase shift can be applied to realize SH beam manipulation with diffraction.However,the realization of full phase control over SH from zero to 2π will be significant for harmonic beam manipulations,such as SH beams bending,intense focusing of SH and so on.Here,we theoretically and experimentally demonstrate the realization of a continuous control over the phase change from zero to 2π for second harmonic generation(SHG)from the complementary split-ring resonators(CSRRs)by adjusting their geometrical parameters and reversing their orientations.It has been discovered that the zero-order resonance and linear phase response of CSRRs arrays can be tailored by the armlength of CSRRs,and the phase of output linear response can be changed simultaneously.We also simulate the phase response of SHG based on a hydrodynamic model of the free electron dynamics and the phase of SHG can also been changed.We combine this phase shift with π phase shift from reversed CSRRs to obtain a complete phase control over the SHG from zero to 2π and large SH amplitudes.We verify this concept by experimentally fabricating modulated arrays of CSRRs with different phase gradients,as well as nonlinear metalens The results match well with the theoretical expectation.The ability to continuously control the SH phase at subwavelength scale is very important for nonlinear nanophotonic devices,such as nonlinear holographic and integrated coherent generation.3.Polarization states control of SH from nonlinear metal metasurfaces.Polarization is one of the important properties of light and has been widely investigated in linear metasurfaces.However,in nonlinear metasurfaces regime,researches on harmonic generations are mainly concentrated on linearly(circularly)polarized harmonics waves generated from linearly(circularly)polarized FW.The key to realize polarization control of SH is to build two orthogonal polarized basic vectors and introduce phase difference between them.We utilize split-ring resonators(SRRs)and complementary split-ring resonators(CSRRs)to provide two orthogonal polarized basic vectors for generated second harmonic(SH)and build the nonlinear metasurfaces,which can realize polarization control of SHG signals.Such two units can generate two orthogonal polarized SH electric field components,and the ratio of the amplitude,as well as phase difference between them,can be tailored by the geometry parameters of CSRRs and SRRs.Additional phase difference can be added into the orthogonal polarized electric field components by introducing spatial offset between the supercells,which are consist of SRRs and CSRRs,respectively.So that we can realize beam splitting and polarization control at the same time.We design dual nonlinear ultrathin quarter-wave plate that can simultaneously generate two separated beams of SH with orthogonal circularly polarizations under the excitation of linearly polarized fundamental wave(FW),based on the nonlinear metasurfaces consists of SRRs and CSRRs.Two orthogonal linearly polarized SH beams are also demonstrated with the dual nonlinear polarization generator,which is illuminated by circularly polarized FW.Furthermore,we show dual elliptical polarization generator with properly designed nonlinear metasurfaces.Our design provides a new approach to realize the polarization control of harmonic generations.4.Enhancement of THG from metal metasurfaces with SPRs.It is known that SPRs can boost the nonlinearity of metal metasurfaces.However,the conversion efficiency still stays low and metal has the disadvantage of low damage threshold.We propose a new way to enhance the THG efficiency of metal metasurfaces by coating a dielectric layer on plasmonic antennas.Due to SPRs,the generated hotspots are mainly localized near the surface of metal and the electric fields inside are actually very low.Thus,the way to enhance the THG efficiency of metal is to enhance the electric fields inside the metal.The proposed gold antennas coating with Al2O3 can enhance the electric fields inside the gold antennas and improve the THG efficiency,comparing with traditional antennas without Al2O3 layer.We believe that the reason for the enhancement of internal field is that the two sides of bare antennas are far away from each other so that they cannot interfere with each other,while the two sides of the gold rod are close to each other,which lead to the interaction between them,thus enhancing the electric field inside the gold rod antenna.Further simulations show that optical antennas with Al2O3 film can realize the enhancement of THG up to 280 times.In addition,the coating layer can increase the damage threshold of metal metasurfaces,and thus the efficiency can be improved with higher excitation power.The mechanism proposed here to enhance THG efficiency provides a new way for improving the efficiencies of harmonic generations,and high efficiencies of harmonic generations can be very useful for coherent light sources and down-conversion. |
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